The present invention relates to a spindle bearing device with two bearing means for mounting a spindle which has a main axis of inertia, the two bearing means forming an axis of rotation. Furthermore, the present invention relates to a corresponding method for mounting a spindle.
When ball-mounted spindles are operated at high rotational speeds, this gives rise to very high undesirable alternating forces on the bearings. The reason for this is the existing unbalance of the rotating spindle rotor. The spindle rotor is a body with a rotationally symmetrical contour, which possesses a geometric axis of symmetry. The geometric axis of symmetry is designated below as the “figure axis”.
It is known from the physics of rotating bodies that any body, even when it is constructed completely asymmetrically, possesses at least three axes, about which it can rotate in a stable manner, without in this case executing disruptive wobbling or vibratory movements. Such an axis is called a main axis of inertia. Rotation about the main axis of inertia is always stable and is maintained without external forces being supplied. This is also the state which is desired when rotating bodies are in operation, since rotation about the main axis of inertia is free of reactions, such as, for example, vibrations or alternating forces, on the bearing points.
The geometric position of a main axis of inertia of a rotor is determined by its mass distribution. If the above-addressed spindle rotor had a mass distribution configured perfectly rotationally symmetrically about the geometric axis of symmetry, the physical main axis of inertia would coincide with the figure axis. This state is aimed at in technology, but can be achieved only up to a certain extent within the scope of the technically and economically implementable possibilities.
The rotor is carried by two ball bearings which are spaced sufficiently far apart from one another on the longitudinal axis of the rotor. The points at which the bearings are located are called the “A-side” and the “B-side” bearing plane. In this case, it is unimportant that, in technically executed spindles, a plurality of individual bearings are often combined into a bearing group and then define the respectively bearing plane. If the outer ring of the bearings is assumed to be anchored firmly in space, then the ball mounting predetermines the later axis of rotation of the rotor in geometric terms. The axis of rotation is then the straight connecting line which connects the A-side and the B-side bearing center to one another. Since the bearing inner ring is designed to be rotationally symmetrical with respect to the bearing center, the axis of rotation of the rotor consequently coincides with its figure axis.
The firm anchoring used according to the prior art for the bearing outer ring places the axis of rotation of the spindle rotor onto its geometric figure axis. However, the physical main axis of inertia will generally not be entirely identical to the rotor axis of rotation imposed by the mounting. For this reason, when the rotor rotates, undesirable vibrations or alternating forces are transmitted to the bearings and their anchoring and increase with a rising rotational speed. These present a problem, particularly in the case of spindles rotating at high speed, and generate disturbing noises, high vibrations and consequently increased wear, thus giving rise to high exchange rates of the bearings which entail costs and standstill times on the machine.
The spindle housing which carries the bearing outer ring is not infinitely rigid, but is anchored to a structure with finite mass and with finite mechanical rigidity. Since the mechanical structure damping is very low, as is known, the anchoring of the spindle housing therefore exhibits pronounced resonances. When the spindle rotor is operated at rotational speeds at which the structural resonance of the anchoring of the spindle housing is excited, then vibrations, bearing forces and noises are intensified considerably due to the insertion of the spindle into the machine structure. Since the manufacturer of the spindles is not aware of the properties of the machine structure into which the spindle will be inserted later, it is virtually impossible for him to give a quantitative forecast about the spindle in the installed state. This is a serious problem for the manufacturers of spindles rotating at high speed which cannot easily be solved.
In order to minimize the abovementioned disturbing vibrations and alternating forces on the bearings and their anchoring, the physical main axis of inertia must be identified as closely as possible with the geometrically imposed axis of rotation. This method is the prior art and is designed as “balancing”. In this case, the mass distribution of the rotor is influenced, and its physical main axis of inertia is thus displaced into the desired position. This takes place by the addition or removal of mass at a suitable point on the rotor. The result of the balancing (correction of the mass distribution) is checked via an evaluation of the oscillation velocity or the bearing forces. For this purpose, there are balancing machines and appliances which indicate the balancing state and calculate suitable stipulation values for the mass correction.
It has been shown that even deviations of the order of only a few micrometers between the orientation of the geometrically imposed axis of rotation and the physical main axis of inertia lead to unacceptably high vibrations. However, in practice, completely exact identity between the geometric axis of rotation and the physical main axis of inertia cannot be achieved even by means of the balancing machines mentioned. There are several reasons for this:                The measurement accuracy of the balancing machines is limited.        The spindle rotor is subject, during operation, to a dimensional variation which is generated by temperature and centrifugal forces. The mass distribution changes as a result, and the physical main axis of inertia then no longer lies on the geometric axis of rotation. It has been shown, above all in spindles rotating at high speed, that the spindle rotor is not completely stable with respect to the centrifugal forces which it itself generates. The form of the rotor, in particular the curvature of the geometric rotor axis, is then dependent on rotational speed.        Particularly in spindles for machine tools, the tools are fastened to the rotor only after the balancing of the rotor, when the machine tool is in operation. The orientation of the physical main axis of inertia is then determined by the distribution of the mass of the rotor and tool. A balancing of the overall rotor/tool system would be required for this purpose, but this is almost always absent for economic reasons.        Tremendous centrifugal forces act on the masses which lie on the outside of rotor. For example, in a spindle rotating at high speed (24000 revolutions per minute), a mass of 1 gram is pulled outward, in the case of radius of 40 mm, with a force which corresponds to a weight of 25 kg. Under the influence of the high centrifugal forces, plastic deformations of parts of the spindle rotor subside only at a very late stage. Consequently, however, the position of the physical main axis of inertia also changes, so that the original balancing state deteriorates.        